Gas sensors for detection of various gases are known from the related art in various designs, functionalities, and detection sensitivities. In a gas sensor, after penetration of gas molecules, a sensor body changes its physical properties such as the electrical resistance or the dielectric constant as a function of the distribution density of the gas molecules. This is analyzed in a system having two electrodes by measuring a resistance or an electrical capacitance.
Such sensors are known, for example, from the publication “Integrated multifunctional humidity sensor” by Krutovertsev in Sensors and Actuators, Vol. A 62 (1997), 582-585, which describes gas sensors having sensitive layers made of various materials and operating as capacitive or resistive sensors. It also describes how the sensitive layers may be manufactured using sol-gel technology.
U.S. Pat. No. 4,768,012 describes a capacitive gas sensor having a sensor body based on silica, titanium, zirconium or aluminum using sol-gel technology. Furthermore, a temperature sensor is integrated into the sensor for compensating for temperature effects in the measurement.
EP 0 403 994 A1 describes a capacitive gas sensor having a polyetherimide layer as a dielectric. Furthermore, the use of cellulose acetate layers as sensitive layers is also mentioned there.
EP 0 403 994 A1 describes a capacitive moisture sensor, i.e., a sensor for water vapor, which is designed as a capacitor in a layered structure. A moisture-sensitive polymer film is provided as a dielectric between two metallic electrodes, one of which is formed by a moisture-permeable metal layer. Depending on the ambient moisture level, more or less water vapor diffuses into the polymer film, thus having a negative effect on its dielectric constant. Measurements of the capacitance of the capacitor formed by the two metal layers and the polymer film therefore allow inferences about the ambient water vapor level. Using such gas sensors based on polymer films, water vapor is detectable in principle in the range from approximately 0% to 100% relative humidity (RH). However, due to the inadequate water vapor sensitivity of the polymer layers, the measurement range below 1% RH is not accessible with sufficient accuracy for sophisticated measurements.
For this reason, more sensitive porous layers, in particular aluminum oxide layers in the case of water vapor, have been used as gas-sensitive layers for a long time. U.S. Pat. No. 2,237,006 thus describes an electric hygrometer in which aluminum oxide as a water vapor-sensitive layer is situated between two metal layers, one of which is permeable to water vapor in a sensor design having a layered structure. The water vapor content, i.e., the moisture, is determined on the basis of the change in the ohmic resistance of this layer induced due to adsorption of water vapor in the aluminum oxide layer.
In comparison with sensors based on polymers, such gas sensors have an expanded detection range. However, their manufacture, in which the porosity of the metal oxide layers is usually created by anodic oxidation of the metals used, is associated with a high manufacturing cost. In addition, they do not have long-term stability and may be used only in a narrow temperature range. Measuring gas temperatures above 100° C. are therefore not accessible to this generic sensor type.
In addition, a moisture sensor in which the electrodes are situated in a layered configuration is also known from KR 00 23937. The water vapor-sensitive layer is applied there with the aid of sol-gel technology. The manufacture of such gas sensors and/or moisture sensors is simplified in comparison with the use of a moisture-permeable cover electrode, i.e., one that is permeable to water vapor. The measurement range extends in a range from approximately 20% to 90% RH in the case of detection of water vapor by the sensors.
In many application cases, the sol-gel technique is preferred in the manufacture of the gas-sensitive layer, which is referred to by the term “sensor body” in conjunction with the present layer because designs other than the design of a planar layer are also conceivable, although the manufacture of sensor bodies is not limited to this technology.
In this sol-gel technique, a colloidal sol is first formed in principle from inorganic salts, organometallic compounds or alkoxides with organic solvents or water and special compounds, in particular stabilizing additives. This sol may be applied to a substrate by various coating operations. For example, it is converted to an amorphous gel by hydrolysis and condensation reactions. This gel is dried and may additionally be processed thermally, e.g., by pyrolysis. It may then be in its oxidic form.
This technique allows comparatively simple mixing of different components of the gas-sensitive layer such as different metal oxides. Furthermore, through suitable sol components and adequate process management, the porosity of the finished gas-sensitive layer and thus its gas adsorption rate and/or gas sensitivity are regulatable to a certain extent. In conjunction with the large contact surface areas between the gas-sensitive layer and the electrodes adjacent thereto in the layered structure in comparison with the structure of intermeshing electrode combs, this yields a highly sensitive gas sensor, which may be used at an operating temperature of up to 300° C. with much higher measuring gas temperatures than known gas sensors based on anodized aluminum (up to 100° C.) or polymers (up to 200° C.).
The electrical impedance of the porous gas-sensitive layer of the sensor is analyzed when using this gas sensor, as is customary with capacitive sensors. The electrical impedance depends on the concentration of the gas to be detected in the surroundings of the gas sensor and/or the amount of gas adsorbed in the gas-sensitive layer. As an alternative to analyzing the impedance of the gas sensor, there is also the possibility of recording only the changes in capacitance or resistance.
As indicated above, the sol-gel technique to be used allows a comparatively simple variation in the constituents of the gas-sensitive layer and variation of the structure of this layer within certain limits. Therefore, in the practical implementation, the composition and structure of the gas-sensitive layer manufactured by the sol-gel technique are tailored to the gas to be detected and to the desired measurement range. In particular, the gas-sensitive layer may be designed specifically for detection of water vapor. In addition, the gas-sensitive layer is designed specifically for detection of traces of gas, in particular trace moisture, i.e., traces of water vapor. Examples of constituents of the sol for such a trace moisture sensor include aluminum, silicon, titanium, magnesium, vanadium, zirconium, barium or iron and/or the oxides thereof. In addition, potassium, lithium, carbon, or tin are also possible constituents. Individual metal oxides as well as mixtures of different metal oxides may be used.
It is also possible for the gas-sensitive layer to have an optimized pore size distribution, in particular pore diameters of less than 1 μm on the average. The pore diameter may also be less than 0.3 μm on the average. The layer thickness of the sensor body, if it has the shape of a planar layer, may be a few μm. A gas-sensitive layer having a total layer thickness of less than 1 μm is particularly advantageous.
Regardless of whether the sensor body or the gas-sensitive layer is manufactured by sol-gel technology or as a polymer film, as a metal, metal oxide, or ceramic body, the physical effect is nevertheless based on the diffusion of gas atoms and/or molecules into a solid body. This diffusion process naturally takes a certain amount of time, which determines the response time of the gas sensor. To accelerate the response, it is possible, for example, to make at least one of the electrodes of the gas sensor permeable for the gas to be detected to allow diffusion of the gas molecules from this side over a large area. Nevertheless, it has been found that the response time of such sensors is not as short as desired.
Accordingly, it would be desirable to improve the response time of gas sensors of the type described above.